U.S. patent application number 12/602341 was filed with the patent office on 2010-06-03 for optical amplifier.
This patent application is currently assigned to TRIMATIZ LTD.. Invention is credited to Yoshiaki Horiuchi, Hiroshi Nagaeda, Yoichi Oikawa, Noriyasu Shiga.
Application Number | 20100134862 12/602341 |
Document ID | / |
Family ID | 40075140 |
Filed Date | 2010-06-03 |
United States Patent
Application |
20100134862 |
Kind Code |
A1 |
Nagaeda; Hiroshi ; et
al. |
June 3, 2010 |
OPTICAL AMPLIFIER
Abstract
A high-speed optical amplifier is considered to be an important
optical device because of an increasing demand of routing, which is
accompanied by an increase in complexity of networks. It is
difficult to satisfy a response performance by related-art
techniques, and there has been a problem in achieving a high-speed
response performance of 10 microseconds or less. An optical
amplifier according to the present invention includes: an input
monitor means 500; an optical amplification means 310 including an
optical amplification medium 300, and a control means 400 for
performing feed-forward control. When the optical amplification
means is controlled by the feed-forward control in response to a
signal of the input monitor means 500, an overshoot signal is
applied as a control signal so that a slow response performance
specific to the optical amplification medium has been improved, and
thereby high-speed response performance has been achieved.
Inventors: |
Nagaeda; Hiroshi;
(Sapporo-shi, JP) ; Oikawa; Yoichi; (Sapporo-shi,
JP) ; Shiga; Noriyasu; (Sapporo-shi, JP) ;
Horiuchi; Yoshiaki; (Sapporo-shi, JP) |
Correspondence
Address: |
OSHA LIANG L.L.P.
TWO HOUSTON CENTER, 909 FANNIN, SUITE 3500
HOUSTON
TX
77010
US
|
Assignee: |
TRIMATIZ LTD.
Ichikawa-shi, Chiba
JP
|
Family ID: |
40075140 |
Appl. No.: |
12/602341 |
Filed: |
May 30, 2008 |
PCT Filed: |
May 30, 2008 |
PCT NO: |
PCT/JP2008/060005 |
371 Date: |
November 30, 2009 |
Current U.S.
Class: |
359/239 ;
359/276; 359/333 |
Current CPC
Class: |
H01S 3/094011 20130101;
H04B 10/296 20130101; H01S 3/06754 20130101; H01S 3/1001 20190801;
H01S 3/1003 20130101; H01S 3/10015 20130101; H01S 3/1305 20130101;
H01S 3/1301 20130101 |
Class at
Publication: |
359/239 ;
359/333; 359/276 |
International
Class: |
H01S 3/13 20060101
H01S003/13; H01S 3/10 20060101 H01S003/10 |
Foreign Application Data
Date |
Code |
Application Number |
May 31, 2007 |
JP |
2007-169730 |
Claims
1. An optical amplifier comprising: an input monitor means; an
optical amplification means; and a control means for performing
feed-forward control in order to control amplification of the
optical amplification means using the input monitor means, and the
optical amplifier using the feed-forward control, wherein an
overshoot signal is used for the amplification control.
2. The optical amplifier according to claim 1, wherein the control
means includes an overshoot generation circuit.
3. The optical amplifier according to claim 2, wherein the
overshoot generation circuit is a PID circuit.
4. The optical amplifier according to claim 1, wherein an amount of
the overshoot signal is 120% or more of a reference value.
5. The optical amplifier according to claim 1, wherein a subsequent
stage of the optical amplification medium includes a high-speed
output variable means.
6. The optical amplifier according to claim 5, further comprising
an output optical monitor means, wherein the high-speed output
variable means is driven using a signal of the output optical
monitor means.
7. The optical amplifier according to claim 6, wherein the
high-speed output variable means is driven by a difference signal
between a signal from the input monitor means and the signal of the
output optical monitor means.
8. The optical amplifier according to claim 5, wherein the
high-speed output variable means is a high-speed variable optical
attenuator.
9. The optical amplifier according to claim 8, wherein the
high-speed variable optical attenuator includes an electro-optic
effect element, and a drive voltage of the high-speed variable
optical attenuator is 40 V or less.
10. The optical amplifier according to claim 2, wherein an amount
of the overshoot signal is 120% or more of a reference value.
11. The optical amplifier according to claim 3, wherein an amount
of the overshoot signal is 120% or more of a reference value.
12. The optical amplifier according to claim 2, wherein a
subsequent stage of the optical amplification medium includes a
high-speed output variable means.
13. The optical amplifier according to claim 3, wherein a
subsequent stage of the optical amplification medium includes a
high-speed output variable means.
14. The optical amplifier according to claim 4, wherein a
subsequent stage of the optical amplification medium includes a
high-speed output variable means.
15. The optical amplifier according to claim 13, further comprising
an output optical monitor means, wherein the high-speed output
variable means is driven using a signal of the output optical
monitor means.
16. The optical amplifier according to claim 14, further comprising
an output optical monitor means, wherein the high-speed output
variable means is driven using a signal of the output optical
monitor means.
17. The optical amplifier according to claim 6, wherein the
high-speed output variable means is driven by a difference signal
between a signal from the input monitor means and the signal of the
output optical monitor means.
18. The optical amplifier according to claim 6 wherein the
high-speed output variable means is a high-speed variable optical
attenuator.
19. The optical amplifier according to claim 7, wherein the
high-speed output variable means is a high-speed variable optical
attenuator.
20. The optical amplifier according to claim 18, wherein the
high-speed variable optical attenuator includes an electro-optic
effect element, and a drive voltage of the high-speed variable
optical attenuator is 40 V or less.
Description
TECHNICAL FIELD
[0001] The present invention relates to an optical amplifier and to
a method of controlling an optical amplifier.
BACKGROUND ART
[0002] In a Wavelength Division Multiplexing (WDM) communication
network, a network configuration is being changed from a
point-to-point network to a ring network using Reconfigurable
Optical Add-drop Multiplexer (ROADM) nodes in a truck line system
and a metro core system.
[0003] A fixed wavelength signal is assigned for add/drop at each
ROADM node, and it has become technically indispensable to remotely
change the number of wavelengths at trouble time, or at the time of
increasing/decreasing a channel.
[0004] In this case, the number of input signals is changed.
Accordingly, for an erbium-doped fiber amplifier (Er-Doped Fiber
Amplifier: EDFA) disposed in an ROADM, it is requested to perform
optical-amplification-gain constant control (Automatic Gain
Control: AGC) such that each signal level will not be changed even
if the number of signals is changed, which is different from
optical-output constant control (Automatic Level Control: ALC)
normally used in a system not using an ROADM.
[0005] For AGC-EDFAs performing this control, amplifiers following
input variations of a few milliseconds have been already in
practical use because the speed of a change in the number of
wavelengths is a few milliseconds.
[0006] In a next-generation ring network, it is requested to
effectively utilize wavelength resources in order to meet an
increase in traffic. Thus, for an ROADM, it is necessary to employ
a dynamic ROADM which is capable of dynamically changing the number
of wavelengths in response to transitional variations of the
communication capacity. In this case, for an AGC-EDFA, it is
necessary to have a response of about 10 microseconds.
[0007] To date, there has been a report on a response of a few
hundred microseconds. However, in order to achieve a higher-speed
ROADM, it is requested to increase the speed further.
[0008] Also, further, in an Optical Burst Switching (OBS) system
dynamically changing paths of a burst signal, it becomes necessary
to have an AGC-EDFA having a response of 10 microseconds or
less.
[0009] In this manner, there have been increasing demands for a
higher speed of 10 microseconds or less, and thus an AGC-EDFA has
become a key device in a next-generation network.
[0010] For a configuration and control of an AGC-EDFA, it is
thought that there are (1) Feedback (FB) control of EDF, (2)
Feed-forward (FF) control of EDF (Japanese Patent No. 3811630), and
(3) Combination control of FF control and FB control.
[0011] Also, for a configuration of an EDFA, there is an example of
a configuration combined with a variable optical attenuator
(Variable Optical Attenuator: VOA) (Japanese Patent Application
Laid-open No. 7-212315). [0012] Patent Document 1: Japanese Patent
No. 3811630 [0013] Patent Document 2: Japanese Patent Application
Laid-open No. 7-212315
DISCLOSURE OF INVENTION
Problems to be Solved by the Invention
[0014] In a method of controlling an EDF, singly, by FF control, FB
control, or the combination control thereof, including Patent
Document 1, it is not possible to eliminate slow recombination
velocity specific to the EDF, and thus it is not suitable for
speeding up. Accordingly, under the related-art driving condition,
there has been a limit to improve transitional response
performance.
[0015] Also, there have been methods of using an EDF and a VOA as
described in Patent Document 2. However, the purpose of a VOA is to
vary a steady output level of an EDF, and controlling of a
transitional response depends only on the control of EDF. Thus, it
has been difficult to make a high-speed response in the same manner
as controlling an EDF singly.
[0016] In view of the above circumstances, the present invention
has been made in order to achieve an improvement in speed of an
AGC-EDFA.
Means for Solving the Problems
[0017] In order to solve the above-described problems, an optical
amplifier according to the present invention achieves high speed by
improving or eliminating slow components of response specific to an
EDF, which is caused by a slow time constant of the recombination
of erbium ions.
[0018] An optical amplifier according to Claim 1 at least includes:
an input monitor means; an optical amplification means; and a
control means for performing feed-forward control in order to
control amplification of the optical amplification means using the
input monitor means, and the optical amplifier using the
feed-forward control, wherein an overshoot signal is used for the
amplification control.
[0019] The "optical amplification means" is an amplification means
which is excited by light or electrons, makes output light strength
greater than input light strength, and includes an optical
amplification medium.
[0020] The "input monitor means" is a means for monitoring an
electronic signal or an optical signal including strength
information of input light into the optical amplifier.
[0021] The "feed-forward control" means control in which a
controlled object is driven by an output control signal in
accordance with a strength of a signal being input into the
controlled object as well.
[0022] As described in FIG. 12, the "overshoot signal" means a
control signal indicating a higher value than a steady level
transitionally. Here, a steady level is control signal strength at
the time when one second or more has passed after the start of
applying the control signal. "Transitionally" means time within 100
microseconds from the start of applying the signal.
[0023] An optical amplifier according to Claim 2 is the optical
amplifier according to Claim 1, wherein the control means includes
an overshoot generation circuit.
[0024] The "overshoot generation circuit" generally means an analog
circuit or a digital circuit which generates the overshoot signal
as an electric circuit.
[0025] An optical amplifier according to Claim 3 is the optical
amplifier according to Claim 2, wherein the overshoot generation
circuit is a PID circuit.
[0026] As shown in FIG. 9, the "PID circuit" is a circuit which
produces the amount of control signal determined by a linear
combination of three components, a proportional element, a
derivation element, and an integral element with respect to an
input signal, and thus is capable of generating any waveform in
response to the input signal.
[0027] An optical amplifier according to Claim 4 is the optical
amplifier according to any one of claims 1 to 3, wherein an amount
of the overshoot signal is 120% or more of a reference value.
[0028] As shown in FIG. 12, a peak level of the control signal is
defined as a maximum value of the overshoot signal. Also, in this
specification, an amount of overshoot signal is defined as a value
produced by dividing a peak level of the control signal by the
steady level. Also, the reference value has a same meaning as the
steady level.
[0029] An optical amplifier according to Claim 5 is the optical
amplifier according to any one of claims 1 to 4, wherein a
subsequent stage of the optical amplification means includes a
high-speed output variable means.
[0030] The "high-speed output variable means" is a means for
outputting input light, varying the output by giving a drive
signal, and having a high-speed performance in responding to
variations of 6 dB before and after giving the drive signal in 10
microseconds or less.
[0031] For the high-speed output variable means, for example, a
semiconductor amplifier (SOA), an optical external modulator, a
high-speed optical variable attenuator, etc., are considered.
[0032] An optical amplifier according to Claim 6 is the optical
amplifier according to Claim 5, which further includes an output
optical monitor means, wherein the high-speed output variable means
is driven using a signal of the output optical monitor means.
[0033] The "output light monitor means" is a means for detecting a
power level of output light using a part of the output light.
[0034] An optical amplifier according to Claim 7 is the optical
amplifier according to Claim 6, wherein the high-speed output
variable means is driven by a difference signal between a signal
from the input monitor means and a signal of the output optical
monitor means. The signal from the input monitor means is a signal
including information of the input light strength, and the signal
of the output optical monitor means is a signal including
information of the output light strength.
[0035] An optical amplifier according to Claim 8 is the optical
amplifier according to any one of claims 5 to 7, wherein the
high-speed output variable means is a high-speed variable optical
attenuator (VOA).
[0036] The "high-speed variable optical attenuator" is a means for
attenuating and outputting input light, varying the amount of
attenuation by giving a drive signal, and having a high-speed
performance in responding to variations of 6 dB before and after
giving the drive signal in 10 microseconds or less.
[0037] An optical amplifier according to Claim 9 is the optical
amplifier according to Claim 8, wherein the high-speed variable
optical attenuator includes an electro-optic effect device, and a
drive voltage of the high-speed variable optical attenuator is 40 V
or less.
[0038] The "electro-optic effect device" means a device having an
electro-optic effect, such as Pockels effect, Kerr effect, etc.
[0039] Also, the "drive voltage is 40 V or less" means that the
voltage difference applied across both ends of the electro-optic
effect device is 40 V or less.
Advantages
[0040] As described above, by the present invention, it becomes
possible to increase a response performance of an optical
amplifier.
BRIEF DESCRIPTION OF THE DRAWINGS
[0041] FIG. 1 is an embodiment diagram of an optical amplifier
using feed-forward control.
[0042] FIG. 2 is a diagram illustrating principles of general
feedback and feedforward control.
[0043] FIG. 3 is a diagram illustrating a principle of overshoot
operation in the feed-forward control according to the present
invention.
[0044] FIG. 4 is a diagram illustrating a relationship between an
amount of overshoot and response speed of EDF.
[0045] FIG. 5 is an embodiment diagram of an optical amplifier
using feed-forward control and feedback control.
[0046] FIG. 6 is a diagram illustrating a principle of an effect of
introducing a high-speed output variable means.
[0047] FIG. 7 is an exemplary diagram of a configuration of a VOA
drive circuit.
[0048] FIG. 8 is an exemplary diagram of a simulation result of the
VOA drive circuit.
[0049] FIG. 9 is an exemplary diagram of a configuration of a PID
circuit.
[0050] FIG. 10 is a response waveform chart of a falling
portion.
[0051] FIG. 11 is a response waveform chart of a rising
portion.
[0052] FIG. 12 is an explanatory diagram of an overshoot
signal.
REFERENCE NUMERALS
[0053] 10 input light
[0054] 20 output light
[0055] 300 optical amplification means
[0056] 310 optical amplification means
[0057] 320, 321 PUMP-LDs
[0058] 330, 331 PUMP-LD drive circuits
[0059] 400 feed-forward control means
[0060] 403, 404, 405 PID circuits
[0061] 410 comparison section
[0062] 420 gain setting terminal
[0063] 430 OPAMP
[0064] 500 input monitor means
[0065] 510, 511 TAP-PDs
[0066] 520, 521 TIAs
[0067] 610 high-speed output variable means
[0068] 620 high-speed variable optical attenuator (VOA)
[0069] 630 VOA drive circuit
[0070] 700 output optical monitor means
[0071] 800 feedback control means
[0072] 810 OPAMP
BEST MODE FOR CARRYING OUT THE INVENTION
[0073] In the following, a description will be given of an
embodiment of the present invention on the basis of the
drawings.
Embodiment 1
[0074] A first embodiment is shown in FIG. 1. An optical amplifier
includes an input monitor means 500, an optical amplification means
310, and a feed-forward control means 400.
[0075] The input monitor means 500 includes a TAP-type light
receiving element 510 and a transimpedance-type electric amplifier
(TIA) 520. Also, a part of input light may be branched by an
optical coupler, and a level of the input light may be monitored.
Moreover, it is possible to input a part of input light branched in
a preceding stage of the optical amplifier into an input monitor
means (photodiode, TIA) of the optical amplifier.
[0076] Also, it is possible to monitor the input light level by
separating supervisor control light using a WDM coupler, and
transferring an electronic signal equivalently representing the
input light level to the input monitor means by the
supervisor-control-light processing section. Further, it is
possible to monitor the input light level by transferring an
electronic signal equivalently representing the input light level
from the supervisor-control-light processing section, which has
already been processed at a preceding stage of the optical
amplifier.
[0077] The optical amplification means 310 uses an EDF as an
optical amplification medium 300, and includes a PUMP-LD 320 and a
PUMP-LD drive circuit 330, and is an example of using the two
PUMP-LDs 320. The optical amplification means includes a
semiconductor optical amplifier device in addition to an EDF.
[0078] In the control of the optical amplification means 300, the
PUMP-LD 320 is feed-forward controlled by the output information of
the input-side TIA 520 in order to drive the optical amplification
means 300 at a high speed in response to a change in the level of
input light. A PID circuit section 403 drives the PUMP-LD 320 by
producing an overshoot signal of the control signal.
[0079] In a differential operational amplifier 430 constituting the
control means, the PUMP-LD can be driven by a requested drive
current by setting a plus-side input, thereby allowing gain
setting.
[0080] Next, a detailed description will be given of speeding up by
feed-forward control using an overshoot signal. First, a
description will be given of general feed-forward control and
feed-back control using FIG. 2.
[0081] In general feed-forward control, a control signal has
nothing but a drive waveform similar to an input signal. Thus, it
is impossible to increase the response of an EDF having a low
response speed.
[0082] On the other hand, in general feed-back control, a
difference between an input-signal waveform and an output-signal
waveform having a slow response becomes a drive waveform, and thus,
in general, becomes a drive waveform having ringing. Accordingly,
the response waveform includes a quickened component, but ringing
remains, and thus it takes some time until the waveform converges
in a stable level. In this control, if control is performed with a
time constant restraining the occurrence of ringing, it becomes
impossible to improve the response speed of the waveform.
[0083] FIG. 3 shows a principle of obtaining a high-speed response
characteristic in feed-forward control by driving an EDF using an
overshoot signal.
[0084] If a control signal from the control means is a rectangular
wave, a response having a slow rising time is produced because an
EDF has a slow component specific to the EDF. A drive signal level
from the control means obtaining a requested EDF response amplitude
is assumed to be level-1, and a drive signal level higher than that
is assumed to be level-2. When driven by the level-2, a large
EDF-response amplitude is produced as shown in FIG. 3.
[0085] If the control signal has a waveform which overshoots
level-2, and then stably falls to level-1, the EDF rises with a
response corresponding to level-2, and then has a response
amplitude corresponding to level-1. Accordingly, it becomes
possible to increase the speed of the EDF by the overshoot.
[0086] FIG. 4 shows a result, obtained by an experiment, of an
amount of overshoot defined by the ratio of a peak level of the
control signal to a steady level, and the ratio of fast component
in the response amplitude of the EDF. By increasing the amount of
overshoot, the response speed of the EDF is improved.
[0087] This shows a phenomenon in which a ratio of slow response
component of the EDF decreases, and the response speed of the EDF
increases. If the amount of overshoot becomes 120% or more, the
response tends to be saturated. This means that if the amount of
overshoot is increased to 120% or more, a sufficient effect of
increasing response can be obtained.
[0088] As shown in FIG. 9, in order to generate the amount of this
overshoot, for example, a PID circuit in which the amount of a
control signal is determined by a linear combination of a
proportional element (P), a derivation element (D), and an integral
element (I) with respect to an input signal, is provided. Thus, any
waveform can be generated with respect to an input signal.
[0089] A requested overshoot control signal is created by suitably
adjusting individual circuit constants of the three elements, and
the control signal of the optical amplification means is made to be
an overshoot signal, so that it becomes possible to improve the
slow response performance specific to the optical amplification
means.
[0090] Further, it is possible to employ a digital method in which
an overshoot waveform responding to input is stored in a memory, an
input monitor value is read, and a necessary overshoot waveform is
drawn from the memory to be converted into a control signal.
[0091] Next, a description will be given of conditions and an
experiment result in the configuration shown in FIG. 1. The
employed EDF has an erbium concentration of 7.9.times.10.sup.24
m.sup.-3 and a length of 15 m, and thus is a typical EDF.
[0092] The wavelength of the PUMP-LD is 0.98 .mu.m, the power of
one unit is +23.5 dBm, and pumping is performed both forward and
backward. In this regard, the current driving the PUMP-LD had a
steady value of 370 mA, and an overshoot current of 140% was used.
The response characteristic of the EDF at that time is the data of
an overshoot of 140% illustrated in FIG. 4. The gain of the entire
amplifier is set to 25 dB.
[0093] A measurement result of the response characteristic on this
condition is the response waveform at the time when an amount of
overshoot is 140% in FIG. 4.
[0094] The measurements were made using a light source having a
wavelength of 1550 .mu.m, and light input having a stepwise
increase of -8 dBm is used on a reference level of -14 dBm. This
becomes 6 dB as a fluctuation band of input light amplitude. This
fluctuation band simulates a state in which the number of WDM input
light into the optical amplifier varies to four times in a stepwise
manner.
[0095] As is understood from the figure, the speed of rising time
defined by 10% to 90% can be increased to 10 microseconds.
Embodiment 2
[0096] A second embodiment is shown in FIG. 5. The optical
amplifier includes an input monitor means 500, an optical
amplification means 310, and a feed-forward control means 400, a
high-speed output variable means 610, an output light monitor means
700, and a feed-back control means 800. The input monitor means 500
includes a TAP-type light receiving element 510 and a
transimpedance-type electric amplifier (TIA) 520.
[0097] The optical amplification means 310 includes an EDF as an
optical amplification medium 300, a PUMP-LD 320 and a PUMP-LD drive
circuit 330, and is an example of using the two PUMP-LDs 320.
[0098] The output-light monitor means 700 has a same configuration
as that of the input-light monitor means 500.
[0099] The high-speed output variable means 610 includes a
high-speed variable optical attenuator (VOA) 620 and a VOA drive
circuit 630. In this regard, a high-speed variable amplifier, such
as a semiconductor amplifier, may be used as a high-speed output
variable means.
[0100] Next, a description will be given of a principle of
achieving high-speed response characteristic using a high-speed
output variable means using FIG. 6. The high-speed output variable
means eliminates a slow response component of the EDF (a portion
enclosed by a quadrilateral in the figure), and shapes the response
waveform into a rectangular wave so that only an originally
fast-rising component of the EDF remains, making it possible to
achieve a response speed of one microsecond or less.
[0101] In the control of the optical amplification means 300, the
PUMP-LD 320 is feed-forward controlled by the output information of
the input-side TIA 520 in order to drive the optical amplification
means 300 at a high speed in response to a change in the level of
input light. A PID circuit section 403 drives the PUMP-LD 320 by
producing an overshoot signal of the control signal.
[0102] In a differential operational amplifier 430 constituting the
control means, the PUMP-LD can be driven by a requested drive
current by setting a plus-side input, thereby allowing the EDF
setting.
[0103] In the control of the high-speed variable optical attenuator
(VOA) 620, the VOA 620 is feed-back controlled by a difference
signal of the output information of the input-side TIA 520 and the
output information of the output-side TIA 521.
[0104] In this regard, the PID circuit section 405 in the feedback
control means 800 can change the drive signal of the VOA 620 into
an overshoot signal.
[0105] Further, it is possible to employ auto-level control, in
which the VOA 520 is feedback controlled such that the output
becomes a constant level on the basis of the output information of
the output-side TIA 521. Also, it is possible to insert a VOA input
light monitor immediately before the VOA 520, and to perform
feed-forward control by a light strength signal obtained from the
VOA input light monitor. Moreover, it is possible to change the
drive signal of the VOA 620 into an overshoot signal in the
feed--forward control of the VOA 520.
[0106] Next, a description is given of a drive circuit of the VOA.
FIG. 7 shows an example of the drive circuit of a VOA. Two same
circuits are vertically disposed, and a drive voltage at a plus
side and a minus side are applied to both ends of the VOA so that a
large amplitude drive is achieved as an example. For devices used,
2SC3423 and 2SA1360 illustrated in the figure are used as
commercially available large current transistors.
[0107] FIG. 8 shows a simulation waveform of a drive waveform in
the present circuit. Note that the result is only of the circuit at
the plus side. The same result having a different polarity is
obtained at the minus side. Thus, the amplitude applied to the VOA
becomes 40 V. From this simulation result, it is understood that a
drive output of one microsecond or less has a limit of 40 V at
most.
[0108] Accordingly, for a VOA, it is understood that it becomes
necessary to have a VOA having a drive voltage of 40 V or less, and
having a high-speed response performance.
[0109] Accordingly, an electro-optic type is promising in that a
VOA of this type has a high-speed performance. Further, for a VOA
using a low voltage of 40 V or less, for example, a PLZT-type
high-speed VOA in international publication NO. WO/2005/121876 is
promising.
[0110] For a VOA, an example of an electro-optic type has been
shown. However, any principle, such as a mechanical type, an MEMS
type, a magneto-optic type, a liquid crystal type, an
electroabsorption type, a thermo-optic type, etc., may be used as
long as a VOA attenuates and outputs input light, varies the amount
of attenuation by being given a drive signal, and has a high-speed
performance in responding to variations of 6 dB before and after
giving the drive signal in 10 microseconds or less.
[0111] With this configuration, the PUMP-LD 320 is driven at a high
speed in response to input light having a rectangular wave, and the
output waveform of the EDF 350 having fast response components and
slow response components is obtained. The slow response-speed
element is removed by the VOA 620, output light 20 is shaped into a
rectangular wave, and a requested high-speed response is
obtained.
[0112] On the other hand, the amount of slow-response components to
be deleted by the VOA 620 decreases, and thus the drive voltage of
the VOA 620 decreases, thereby the response speed of the VOA 620
can be increased. BY these synergistic effects, it becomes possible
to shape an output waveform, and to obtain a response of one
microsecond or less.
[0113] A description will be given of conditions and an experiment
result in the configuration shown in FIG. 5. The employed EDF has
an erbium concentration of 7.9.times.10.sup.24 m.sup.-3 and a
length of 15 m, and thus is a typical EDF.
[0114] The wavelength of the PUMP-LD is 0.98 .mu.m, the power of
one unit is +23.5 dBm, and pumping is performed both forward and
backward. In this regard, the current driving the PUMP-LD had a
steady value of 370 mA, and an overshoot current of 140% was used.
The response characteristic of the EDF at that time is the data of
an overshoot of 140% illustrated in FIG. 4.
[0115] The VOA having a drive voltage amplitude of 30 V and a
response speed of one microsecond or less is applied. In this
regard, the gain of the entire amplifier is set to 20 dB.
[0116] Measurement results of the response characteristic on this
condition are shown in FIG. 10 and FIG. 11. FIG. 10 shows a
response characteristic in a falling section, and FIG. 11 shows a
response characteristic in a rising section.
[0117] The measurements were made using a light source having a
wavelength of 1550 .mu.m, and three kinds of input, light input
having stepwise increases of -5 dBm, -8 dBm, and -11 dBm are used
to a reference level of -14 dBm. This becomes individually 9 dB, 6
dB, and 3 dB as fluctuation bands of an input light amplitude,
which correspond to response waveform levels in the figure. The
fluctuation bands simulate states in which the number of WDM input
light into the optical amplifier varies to eight times, four times,
and two times in a stepwise manner.
[0118] As is understood from the figure, in the individual
input-light fluctuation bands, high-speed characteristics of one
microsecond or less are obtained for both the rising response time
and the falling response time.
[0119] In this regard, the present invention is not limited to the
above-described embodiments. It is possible to make variations
without departing from the spirit and scope of the invention.
INDUSTRIAL APPLICABILITY
[0120] By the present invention, it is possible to achieve an
optical amplifier capable of high-speed response, and thus it is
possible to use the optical amplifier as an optical communication
apparatus.
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